Origin of Solar Wind Periodic Density Structures and Effects on the Earth’s magnetosphere: a case study

In the solar wind, periodic density fluctuations are often present on time scales ranging from a few minutes to a few hours which we refer to as Periodic Density Structures (PDSs). The PDSs belong to the class of “meso-scale structures” with time scales and length scales smaller than those of Coronal Mass Ejections (CMEs) and larger than those of turbulence dissipation scales. Our goal is to correctly characterize the periodicity of these transients as it represents: (I) an important constraint for theories that aim to explain the origin of the solar wind; (II) an important driver of the Earth’s magnetosphere dynamics driving Ultra Low Frequency (ULF) waves with timescales greater than ≈4 minutes that are relevant for radiation belt dynamics and electron precipitation. For this purpose, we have developed a robust spectral analysis procedure for the identification of periodic fluctuations in time series. We applied this procedure to identify fluctuations at specific frequencies in both the solar wind density and magnetospheric field observations during the interaction of the magnetosphere with two interplanetary shocks followed by a train of 90-minute solar wind PDSs on November 9, 2002. Using the Wang-Sheeley-Arge model we identified that the source of this solar wind stream was an active region and a mid-latitude coronal hole just prior to crossing of the Heliospheric Current Sheet. Using combination of satellite and ground magnetometer observations we determined that the magnetospheric response was characterized by: (i) forced breathing by periodic solar wind dynamic pressure variations below ≈1 mHz; (ii) a combination of directly driven oscillations and wave modes triggered by additional mechanisms between ≈1 and ≈4 mHz.